1. Technical Field
This invention relates to compound helicopters.
2. Description of the Prior Art
A compound helicopter usually has a main sustaining rotor for hover and low speed flight, fixed wings to provide lift during high speed forward flight and usually, auxiliary propulsion means to provide additional thrust required for high speed forward flight.
GB-A-1095573, US-A-3138349 and US-A-3241791 all provide a compound helicopter having a rearward facing propeller at the end of a tail boom. The propeller is mounted in a duct and a plurality of vertical vanes in the propeller slipstream provide control surfaces to control forward flight direction and provide anti-torque control when hovering. There is no disclosure of any means or the desirability of varying the ratio of power applied during operation as between the main sustaining rotor and the propeller and it is likely therefore that the forward speed of the disclosed compound helicopter would be limited by the high tip mach number encountered by the advancing blades of the main sustaining rotor as forward speed increases and/or the modest power level which can be absorbed efficiently by the propeller.
Consequently, the installed power is sized for the maximum requirement in hover and may be under-utilised in high speed flight so that in that mode overall efficiency is adversely affected since the helicopter carries power output capacity that cannot be used to advantage in high speed flight.
GB-A-2130984 describes a compound helicopter having a variable cycle gas turbine power plant provided with a variable area final propulsion nozzle which receives the exhaust from a power turbine. In such an arrangement the power supplied to a main sustaining rotor can be reduced during high speed flight, and auxiliary propulsion means in the form of a jet thrust from the nozzle in forward flight is obtained. The jet nozzle may not be fuel efficient when operated in a continuous propulsion mode and, although the rotor may be slowed, rotor aerodynamic boundaries may again be reached as forward speed increases. The engine cannot be operated to provide jet thrust only and will always provide some shaft power output which is likely to be in excess of that required to drive the slowed sustaining rotor in high speed flight so that the available power capacity may again be under-utilised in that mode of operation. This again reduces the overall efficiency of the helicopter. Furthermore, the helicopter will require additional means such as a tail rotor to provide anti-torque forces. A tail rotor is operationally redundant in such an application during high speed flight and is detrimental to performance due to its high drag load and the fact that it consumes power.
An objective of this invention is to provide a compound helicopter capable of high forward speed. A further objective is to provide such a compound helicopter in which installed power is more fully utilised throughout all phases of operation than has been possible heretofore in order to increase the overall efficiency of operation.